1. Field of the Invention
The field of the invention is the electrical motor art, and in particular means for supplying multi-phase excitation to poly-phase brushless motors.
2. Description of Related Art
This invention originated from a need to develop an inexpensive brushless direct current motor controller for a space station experiment pointing system. A brushless D.C. motor can be commutated using several different methods. The first method, six-step commutation, causes an undesirable torque ripple in the pointing system, while the second method, sinusoidal commutation, does not. Because the torque ripple would cause an undesirable jitter in the pointing control, sinusoidal commutation is preferred. Problems arise when the number of poles becomes excessive, as for example with a 48 pole, 3-phase brushless D.C. motor. The back electromotive force (EMF) of a permanent magnet brushless motor will cycle sinusoidally, and the number of cycles is equal to one-half the number of poles per mechanical revolution of the motor. To minimize the torque ripple, the current applied to the motor must be in phase with the motor back EMF. (For a complete description of the four quadrant control of a brushless 3-phase motor , see U.S. Pat. No. 4,644,234.) As shown in the brushless commutation literature, if the current and back EMF are in phase, then the following relationship for torque is valid: EQU T=[sin.sup.2 (.THETA.)+sin.sup.2 (.THETA.+2.pi./3)+sin.sup.2 (.THETA.+4.pi./3)].multidot.K.sub.m .multidot.I=1.5.multidot.K.sub.m .multidot.I
where
T=total motor torque; PA1 K.sub.m =torque constant of the motor; PA1 I=motor current; and PA1 .THETA.=angle of rotor magnets with respect to the stator windings
To generate the sinusoidally varying current, an angular position sensor which has the same frequency and is in phase with the motor back EMF has traditionally been required. The previous method for converting sinusoidal commutations of multi-pole D.C. brushless motors was to employ a multi-speed resolver. For the motor selected, a 48-pole (24-speed) unit, a 3-phase resolver would be required to perform the commutation. This type of resolver is commercially available, but not as an off-the-shelf item. To custom manufacture one would be prohibitively expensive and time consuming, and would have undesirable size and weight factors for space applications. There is a need for an inexpensive solution to this problem. This invention is directed to a solution of these and related problems.